Sheet stacking device and image forming apparatus including the same

ABSTRACT

A sheet stacking device includes a sheet stacking section, pairs of ejection rollers, an airflow generator, and an airflow guide. The airflow guide has a center exhaust port and paired side exhaust ports. An airflow is blown out from the center exhaust port toward a central part of the lower surface of the sheet in a sheet width direction perpendicular to a conveyance direction of the sheet. Airflows are blown out from the respective paired side exhaust ports toward respective sides of the lower surface of the sheet. An amount of the airflow blown out from the center exhaust port is larger than those of the airflows blown out from the respective paired side exhaust ports.

INCORPORATION BY REFERENCE

The present application claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2016-225060, filed on Nov. 18, 2016. The contents of this application are incorporated herein by reference in their entirety.

BACKGROUND

The present disclosure relates to a sheet stacking device for sheet stacking and an image forming apparatus including the sheet stacking device.

A sheet stacking device is known in general that ejects a sheet after image formation onto an exit tray. An image forming apparatus is also known that includes a sheet stacking device including a mechanism that blows wind toward a sheet that is being ejected.

SUMMARY

A sheet stacking device according to an aspect of the present disclosure includes a sheet stacking section, pairs of ejection rollers, an airflow generator, and an airflow guide. A sheet is stacked on the sheet stacking section. The pairs of ejection rollers eject the sheet toward the sheet stacking section. The airflow generator generates airflows. The airflow guide guides the airflows toward a lower surface of the sheet ejected from the pair of ejection rollers toward the sheet stacking section. The airflow guide has a center exhaust port and paired side exhaust ports. The center exhaust port is located downstream of the pairs of ejection rollers. An airflow generated by the airflow generator is blown out from the center exhaust port toward a central part of the lower surface of the sheet in a sheet width direction perpendicular to a conveyance direction of the sheet. The paired side exhaust ports are located on respective opposite sides of the central exhaust port in the sheet width direction. Airflows generated by the airflow generator are blown out from the respective paired side exhaust ports toward respective sides of the lower surface of the sheet. An amount of the airflow blown out from the center exhaust port is larger than those of the airflows blown out from the respective paired side exhaust ports.

An image forming apparatus according to another aspect of the present disclosure includes an image forming section and the above sheet stacking device. The image forming section forms an image on the sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view illustrating an outer appearance of an image forming apparatus including a sheet stacking device according to an embodiment of the present disclosure.

FIG. 2 is a cross-sectional view schematically illustrating an internal configuration of a main unit of the image forming apparatus.

FIG. 3 is an enlarged perspective view schematically illustrating an internal configuration of the sheet stacking device.

FIG. 4 is a side view illustrating a configuration of the sheet stacking section and a vicinity thereof.

FIG. 5 is a schematic diagram illustrating a configuration of an airflow generator and a duct.

FIG. 6 is an enlarged cross-sectional view of the sheet stacking device in a state in which an airflow is blown toward a sheet.

FIG. 7 is a perspective view of the sheet in a situation in which airflows are blown toward the sheet.

FIG. 8 is an enlarged cross-sectional view of the sheet stacking device in a situation in which failure in sheet ejection occurs.

DETAILED DESCRIPTION

The following describes an embodiment of the present disclosure in detail with reference to the accompanying drawings. FIG. 1 is a perspective view illustrating an outer appearance of an image forming apparatus S including a post-printing processing unit 5 according to an embodiment of the present disclosure. FIG. 2 is a cross-sectional view schematically illustrating an internal configuration of a main unit 1 of the image forming apparatus S. The main unit 1 of the image forming apparatus S refers to a so-called in-body ejection type monochrome copier. The main unit 1 of the image forming apparatus S may be a color copier, a printer, a facsimile machine, or a multifunction peripheral. The multifunction peripheral has any of functions of a color copier, a printer, and a facsimile machine.

As illustrated in FIG. 1, the image forming apparatus S includes the main unit 1 and the post-printing processing unit 5. The main unit 1 performs image formation on a sheet P. The post-printing processing unit 5 is disposed next to the main unit 1. The post-printing processing unit 5 includes a post-printing processing section. The post-printing processing section performs specific post-printing processing on a sheet P or a sheet set (sheet sheaf) subjected to image formation. The post-printing processing refers to for example at least one of punching, stapling, folding, and aligning. Punching refers to processing to punch a binding hole in the sheet P. Stapling refers to processing to staple the sheet set. Folding refers to processing to fold the sheet P. Aligning refers to processing to shift or align the sheet P in width.

The main unit 1 includes a main unit casing 100, an image scanning section 2 a, an auto document feeder (ADF) 2 b, and an operation section 10. The image scanning section 2 a is disposed above the main unit casing 100. The auto document feeder 2 b is disposed on the upper surface of the image scanning section 2 a. The operation section 10 is mounted on a front surface of the image scanning section 2 a. The main unit casing 100 accommodates in the interior thereof a sheet feed section 3 a, a conveyance path 3 b, an image forming section 4 a, a fixing section 4 b, and a sheet ejecting section 3 c (see FIG. 2).

The operation section 10 receives user input for various settings and operation instructions to the image forming apparatus S. The operation section 10 includes a numeric keypad 11, a start key 12, and a touch panel 13. The numeric keypad 11 receives numeric input. The numeric keypad 11 receives for example setting of the number of copies. The start key 12 receives input of an instruction to execute a copy operation. The touch panel 13 displays various operation keys and guidance.

The auto document feeder 2 b automatically feeds a document sheet that is a copy target toward a predetermined first document scanning position. The predetermined first document position is located at a position where first contact glass 24 is mounted. In a situation in which a user loads a document sheet on a predetermined second document scanning position using the hand, the auto document feeder 2 b is opened upward. The predetermined second document position is located at a position where second contact glass 25 is mounted. The auto document feeder 2 b includes a document teed tray 21, a document conveyance section 22, and a document exit tray 23. The document sheet is loaded on the document feed tray 21. The document conveyance section 22 conveys the document sheet via an auto document scanning position. The document sheet after being scanned is ejected onto the document exit tray 23.

The image scanning section 2 a has a box casing shape. The first contact glass 24 and the second contact glass 25 are fitted in the upper surface of the image scanning section 2 a. The first contact glass 24 is glass for scanning the document sheet automatically fed from the auto document feeder 2 b. The second contact glass 25 is glass for scanning the document sheet loaded using the hand. The image scanning section 2 a optically scans an image of the document sheet.

As illustrated in FIGS. 1 and 2, the sheet feed section 3 a in the main unit casing 100 includes a plurality of cassettes 31. The cassettes 31 include for example a first cassette 31A, a second cassette 31B, a third cassette 31C, and a fourth cassette 31D arranged in four stages. The first to fourth cassettes 31A to 31D are disposed in descending order from above. The cassettes 31 each accommodate sheets P having a specific size. The specific size refers to for example A4 size or B4 size. Alternative examples of the specific size include A-type sheet size such as A4 size and B-type sheet size such as B4 size. The cassettes 31 each accommodate a specific type of sheets P. The specific type of sheets P refers to for example copy paper, recycled paper, thick paper, or viewgraphs. A plurality of sheet feed rollers 32 that are driven to rotate are each provided for a corresponding one of the cassettes 31. The sheet feed rollers 32 include for example a first sheet feed roller 32A, a second sheet feed roller 32B, a third sheet feed roller 32C, and a fourth sheet feed roller 32D. The first sheet feed roller 32A feeds one at a time the sheets P accommodated in the first cassette 31A to the conveyance path 3 b. The second sheet feed roller 32B feeds one at a time the sheets P accommodated in the second cassette 31B to the conveyance path 3 b. The third sheet feed roller 32C feeds one at a time the sheets P accommodated in the third cassette 31C to the conveyance path 3 b. The fourth sheet feed roller 32D feeds one at a time the sheets P accommodated in the fourth cassette 31D to the conveyance path 3 b.

The conveyance path 3 b is a conveyance path through which the sheet P is conveyed in the main unit casing 100 from the sheet feed section 3 a to an in-body ejection tray 33. The conveyance path 3 b is also a conveyance path through which the sheet P is conveyed in the main unit casing 100 from the sheet feed section 3 a to the post-printing processing unit 5. A guide plate, conveyance roller pairs 34, and a registration roller pair 35 are provided on the conveyance path 3 b. The guide plate guides the sheet P. The conveyance roller pairs 34 rotate to convey the sheet P toward the image forming section 4 a. The conveyance roller pairs 34 include a plurality of conveyance roller pairs such as a first conveyance roller pair 34A, a second conveyance roller pair 34B, and a third conveyance roller pair 34C. The registration roller pair 35 temporarily stops the sheet P before the image forming section 4 a and feeds the sheet P to the image forming section 4 a in synchronization with toner image transfer.

The image forming section 4 a generates a toner image and transfers the toner image onto the sheet P. That is, the image forming section 4 a forms an image on the sheet P. The image forming section 4 a includes a photosensitive drum 41, a charger 42, an exposure device 43, a developing device 44, a transfer roller 45, and a cleaner 46. The charger 42, the exposure device 43, the developing device 44, the transfer roller 45, and the cleaner 46 are disposed around the photosensitive drum 41.

The photosensitive drum 41 forms an electrostatic latent image and an toner image on the circumferential surface of the photosensitive drum 41 by rotating about a shaft of the photosensitive drum 41. The charger 42 uniformly charges the surface of the photosensitive drum 41. The exposure device 43 includes a laser light source and an optical device such as a mirror or a lens. The exposure device 43 irradiates the circumferential surface of the photosensitive drum 41 with a laser beam L based on image data of the image of the document sheet to form an electrostatic latent image. The developing device 44 supplies toner to the circumferential surface of the photosensitive drum 41 to develop the electrostatic latent image formed on the photosensitive drum 41. The transfer roller 45 forms a transfer nip part in cooperation with the photosensitive drum 41 into a toner image. Transfer bias is applied to the transfer roller 45. The toner image on the photosensitive drum 41 is transferred to the sheet P passing through the transfer nip part. The cleaner 46 includes a cleaning roller and cleans the circumferential surface of the photosensitive drum 41 after toner image transfer.

The fixing section 4 b fixes to the sheet P the toner image having been transferred to the sheet P. The fixing section 4 b includes a heating roller 47 and a pressure roller 48. The heating roller 47 includes a heating element therein. The pressure roller 48 is in press contact with the heating roller 47 to form a fixing nip. When the sheet P to which the toner image has been transferred passes through the fixing nip, toner of the toner image is heated and melt. As a result, the toner image is fixed to the sheet P. The sheet P having being subjected to fixing is fed to the sheet ejecting section 3 c.

The sheet ejecting section 3 c includes an out-body ejection roller pair 36A and an in-body ejection roller pair 36B. The out-body ejection roller pair 36A feeds the sheet P subjected to image formation toward the post-printing processing unit 5. The in-body ejection roller pair 36B feeds the sheet P subjected to image formation toward the in-body ejection tray 33. The out-body ejection roller pair 36A and the in-body ejection roller pair 36B each are driven to rotate in an ejection operation to eject the sheet out of the main unit casing 100. The sheet ejecting section 3 c further includes a switch lever 37 that switches a feed direction of the sheet P. The switch lever 37 turns to guide the sheet P toward an ejection destination specified through the operation section 10.

The post-printing processing unit 5 (sheet stacking device) stacks the sheet P after performing specific post-printing processing. The post-printing processing unit 5 includes a post-printing processing unit casing 500 (casing) and a post-printing processing section. The post-printing processing unit casing 500 is disposed next to the main unit casing 100. The post-printing processing section is disposed in the interior of the post-printing processing unit casing 500 and performs the post-printing processing on the sheet P. FIG. 3 is an enlarged perspective view schematically illustrating an internal configuration of the post-printing processing unit 5.

The post-printing processing unit 5 includes a shaft 51, ejection rollers 52 (ejection roller pairs), driven rollers 53 (upper ejection rollers), a main exit tray 54 (sheet stacking section), and a sub-exit tray 55.

The post-printing processing unit casing 500 has a side surface facing the main unit casing 100 and having an unillustrated conveyance inlet. The post-printing processing unit casing 500 receives the sheet P subjected to image formation through the conveyance inlet. The post-printing processing unit casing 500 has a left surface having a main conveyance outlet and a sub-conveyance outlet from each of which the sheet P is ejected out of the post-printing processing unit casing 500. The left side surface of the post-printing processing unit casing 500 is located opposite to the side surface facing the main unit casing 100. The main exit tray 54 and the sub-exit tray 55 are mounted in correspondence with the main conveyance outlet and the sub-conveyance outlet, respectively, on the left side surface of the post-printing processing unit casing 500 (see FIG. 1).

The main exit tray 54 is mounted on the post-printing processing unit casing 500. The main exit tray 54 is a tray on which a sheet P or a sheet set is stacked. The sheet P or the sheet set is for example subjected to stapling, shifting, and width aligning and then ejected onto the main exit tray 54 by the ejection rollers 52. The main exit tray 54 has an inclined surface 54 a ascending downstream in a conveyance direction DP of the sheet P. The inclined surface 54 a increases in height downstream in the conveyance direction DP of the sheet P.

The sub-exit tray 55 is a tray on which a sheet P ejected from the sub-conveyance outlet is stacked. The sub-exit tray 55 is spaced above the main exit tray 54. A sheet P having been conveyed into the post-printing processing unit casing 500 is selectively ejected onto either the sub-exit tray 55 or the main exit tray 54. For example, either or both a sheet P not subjected to any post-printing processing in the post-printing processing unit 5 and a sheet P subjected to only punching are stacked on the sub-exit tray 55.

The ejection rollers 52 are supported to the post-printing processing unit casing 500 in a rotatable manner. The ejection rollers 52 are supported through the shaft 51 in the present embodiment. The shaft 51 is supported to the post-printing processing unit casing 500 through a unillustrated bearing. The ejection rollers 52 convey in a predetermined conveyance direction (leftward) the sheet P having been conveyed into the post-printing processing unit casing 500 from the sheet ejecting section 3 c of the main unit 1. The ejection rollers 52 eject the sheet P toward the main exit tray 54. Specifically, the ejection rollers 52 eject the sheet P, which has been conveyed into the post-printing processing unit casing 500 from the sheet ejecting section 3 c of the main unit 1, toward the main exit tray 54. Note that the ejection rollers 52 include four ejection rollers 52 spaced from one another in a sheet width direction (front-back direction) W in the present embodiment, as illustrated in FIG. 3. It is only required in the present disclosure that at least two ejection rollers 52 are provided. Four or more ejection rollers may be disposed in an alternative embodiment. The ejection rollers 52 in plural number are spaced from one another in the sheet width direction W. The ejection rollers 52 includes two ejection rollers (paired inside ejection rollers) 52 a and paired ejection rollers (paired outside ejection rollers) 52 b. The two ejection rollers 52 a are disposed between the paired ejection rollers 52 b. The paired ejection rollers 52 b include a first ejection roller (first lower ejection roller) 521 and a second ejection roller (second lower ejection roller) 522. The sheet width direction W refers to a direction perpendicular to the conveyance direction DP (see FIG. 6) of the sheet P ejected from the ejection rollers 52 (nip parts 56) toward the main exit tray 54. The sheet width direction W is parallel to the sheet surface P1.

The driven rollers 53 are supported to the post-printing processing unit casing 500 on the ejection rollers 52 in a rotatable manner. The driven rollers 53 are supported to the post-printing processing unit casing 500 on the ejection rollers 52 in a rotatable manner. Respective nip parts 56 through which the sheet P passes are formed between the driven rollers 53 and the ejection rollers 52. Some of the ejection rollers 52 are each opposite to a corresponding one of the driven rollers 53. The nip parts 56 are formed between the ejection rollers 52 and the driven rollers 53 opposite thereto. Two nip parts 56 are formed in the present embodiment. The driven rollers 53 are spaced from one another in the sheet width direction W. In the present embodiment, the driven rollers 53 are spaced from one another in the front-back direction. The respective nip parts 56 are formed between the ejection rollers 52 and the driven rollers 53 opposite thereto. The sheet P passes through the nip parts 56. Specifically, the sheet P passes through the nip parts 56 and is ejected toward the main exit tray 54.

An unillustrated conveyance roller and an unillustrated driven roller each for ejecting the sheet P onto the sub-exit tray 55 are further provided in the interior of the post-printing processing unit casing 500.

The post-printing processing unit 5 further includes a first fan 71 (airflow generator, first airflow generator), a second fan 72 (airflow generator, second airflow generator), and a duct 73 (airflow guide). FIG. 4 is a side view illustrating a configuration of the main exit tray 54 of the post-printing processing unit 5 and the vicinity thereof. FIG. 5 is a schematic diagram illustrating a configuration of the first and second fans 71 and 72 and the duct 73. FIG. 6 is an enlarged cross-sectional view of the post-printing processing unit 5 in a state in which airflows DF are blown toward the sheet P.

The first and second fans 71 and 72 each generate an airflow DF blowing toward the sheet P. The first and second fans 71 and 72 each are a known sirocco fan in the present embodiment. As illustrated in FIGS. 5 and 6, the first and second fans 71 and 72 are disposed below the ejection rollers 52 in the post-printing processing unit casing 500 and spaced from each other in the front-back direction in the present embodiment. Note that the first fan 71 disposed in front is illustrated in FIG. 6. The second fan 72 is spaced from the first fan 71 in the rear of the first fan 71 in FIG. 6. The first and second fans 71 and 72 each have an air outlet open upward.

The duct 73 is disposed in the interior of the post-printing processing unit casing 500. The duct 73 communicates the first and second fans 71 and 72 with a space above the main exit tray 54. The duct 73 guides the airflows DF toward the downwardly facing sheet surface P1 of the sheet P ejected from the ejection rollers 52 (nip parts 56) toward the main exit tray 54. The sheet surface P1 is a surface of the sheet P that faces downward.

The duct 73 has a center exhaust port 61 and parried side exhaust ports 62 and includes a first duct 73J and a second duct 73K. The center exhaust port 61 and the side exhaust ports 62 are located below the ejection rollers 52, as illustrated in FIGS. 3 and 4. The center exhaust port 61 and the side exhaust ports 62 are located above the base end of the main exit tray 54. The center exhaust port 61 and the side exhaust ports 62 are located in the left side surface of the post-printing processing unit casing 500. The center exhaust port 61 and the side exhaust ports 62 are aligned with one another in the sheet width direction W. The center exhaust port 61 is located between the side exhaust ports 62. The center exhaust port 61 and the side exhaust ports 62 are located below the ejection rollers 52.

An airflow D61 is blown out from the center exhaust port 61 toward a central portion P11 of the sheet surface P1 of the sheet P, which is ejected by the ejection rollers 52, in the sheet width direction (front-back direction) W. The side exhaust ports 62 are located on respective opposite sides of the center exhaust port 61 in the sheet width direction W. The side exhaust ports 62 are spaced from each other in the sheet width direction W. Airflows D62 are blown out from the respective side exhaust ports 62 toward respective sides of the sheet surface P1 of the sheet P in the sheet width direction W. One of the airflows D62 is blown out from a corresponding one of the side exhaust ports 62 toward a part of the sheet surface P1 of the sheet P ejected by the ejection rollers 52 that is located on one side thereof in the sheet width direction W. The other of the airflows D62 is also blown out from the other of the side exhaust ports 62 toward another part of the sheet surface P1 of the sheet P ejected by the ejection rollers 52 that is located on the opposite side thereof in the sheet width direction W. The respective airflows D62 are blown out in substantially the same direction from one (first side exhaust port 621) and the other (second side exhaust port 622) of the side exhaust ports 62. When the center exhaust port 61 and the side exhaust ports 62 are viewed from above, a direction in which the airflow D61 is blown out from the center exhaust port 61 and directions in which the airflows D62 are blown out from the respective side exhaust ports 62 are parallel to the conveyance direction DP of the sheet P. However, as will be described later, the direction in which the airflow D61 is blown out from the center exhaust port 61 and the directions in which the airflows D62 are blown out from the respective side exhaust ports 62 each cross the conveyance direction DP of the sheet P at a predetermined angle as viewed in a horizontal direction (front-back direction).

Positions of the center exhaust port 61 and the side exhaust ports 62 will be described further in detail below with reference to FIG. 4. As described above, the four ejection rollers 52 are provided in the present embodiment. The center exhaust port 61 is located below a region E between the two ejection rollers 52 a. The side exhaust ports 62 include the first side exhaust port 621 and the second side exhaust port 622. The side exhaust ports 62 are each located below a corresponding one of the ejection rollers 52 b of the pair of ejection rollers 52 b. Specifically, the first side exhaust port 621 is located blow the first ejection roller 521 and the second side exhaust port 622 is located blow the second ejection roller 522.

The side exhaust ports 62 each have an outside edge in the sheet width direction W that is substantially aligned in the sheet width direction W with an outside edge of a corresponding one of the ejection rollers 52 b of the pair of ejection rollers 52 b in the sheet width direction W. The side exhaust ports 62 each have an inside edge in the sheet width direction W that is located more inside in the sheet width direction W than an inside edge of a corresponding one of the ejection rollers 52 b of the pair of ejection rollers 52 b in the sheet width direction W. In other words, as illustrated in FIG. 4, the side exhaust ports 62 each extend inside of a corresponding one of the ejection rollers 52 b of the pair of ejection rollers 52 b in the sheet width direction W. Specifically, the first side exhaust port 621 has a first outside edge L1 located at an outside end thereof in the sheet width direction W and a first inside edge M1 located at an inside end thereof in the sheet width direction W. The second side exhaust port 622 has a second outside edge L2 located at an outside end thereof in the sheet width direction W and a second inside edge M2 located at an inside end thereof in the sheet width direction W. The first ejection roller 521 has a third outside edge Q1 located at an outside end thereof in the sheet width direction W and a third inside edge R1 located at an inside end thereof in the sheet width direction W. The second ejection roller 522 has a fourth outside edge Q2 located at an outside end thereof in the sheet width direction W and a fourth inside edge R2 located at an inside end thereof in the sheet width direction W. The first outside edge L1 is aligned in the sheet width direction W with the third outside edge Q1. The second outside edge L2 is aligned in the sheet width direction W with the fourth outside edge Q2. The first inside edge M1 is located inside of the third inside edge R1 in the sheet width direction W. The second inside edge M2 is located inside of the fourth inside edge R4 in the sheet width direction W. The phrase outside in the sheet width direction W refers to a location close to a specific point in the sheet width direction W. The phrase inside side in the sheet width direction W refers to a location away from the specific point in the sheet width direction W. The specific location refers to a central part of a region between the two nip parts 56 in the sheet width direction W.

The first duct 73J divides an airflow generated by the first fan 71 into two airflows and guides the respective airflows to the center exhaust port 61 and one (second side exhaust port 622) of the side exhaust ports 62.

The second duct 73K divides an airflow generated by the second fan 72 into two airflows and guides the respective airflows to the center exhaust port 61 and the other (first side exhaust port 621) of the side exhaust ports 62.

Respective parts of the airflows generated by the first and second fans 71 and 72 are merged together. Accordingly, the amount of the airflow D61 blown out from the center exhaust port 61 is larger than those of the airflows D62 blown out from the respective side exhaust ports 62. As such, the amount of the airflow D61 blown out from the center exhaust port 61 is set larger than those of the airflows D62 blown out from the respective side exhaust ports 62. Specifically, the amount of the airflow D61 blown out from the center exhaust port 61 is set larger than that of the airflow D62 blown out from the first side exhaust port 621 and that of the airflow D62 blown out from the second side exhaust port 622.

The airflow generated by the first fan 71 and the airflow generated by the second fan 72 are each blown out from the center exhaust port 61 and a corresponding one of the side exhaust ports 62 in the present embodiment. Specifically, the airflow generated by the first fan 71 and the airflow generated by the second fan 72 are each blown out leftward from the center exhaust port 61 and a corresponding one of the side exhaust ports 62. The airflow generated by the first fan 71 and the airflow generated by the second fan 72 are blown at specific angles relative the conveyance direction DP of the sheet P. The opening length of the center exhaust port 61 in the sheet width direction (front-back direction) W is double the opening length of each of the side exhaust ports 62 in the sheet width direction W. The opening length in a height direction (up-and-down direction) of the center exhaust port 61 is a half of the opening length of each of the side exhaust ports 62 in the height direction. The center exhaust port 61 has an upper edge aligned in the height direction with the upper edges of the respective side exhaust ports 62. The center exhaust port 61 has a lower edge located above the lower edges of the respective side exhaust ports 62. In the above configuration, the amount of the airflow D61 blown out from the center exhaust port 61 is larger than the amounts of the airflows D62 blown out from the respective side exhaust ports 62.

Specifically, the center exhaust port 61 and the side exhaust ports 62 are directed (see FIG. 3) such that a blowing direction of the airflow D61 blown out from the center exhaust port 61 and blowing directions of the airflows D62 blown out from the respective side exhaust ports 62 ascend downstream in the conveyance direction DP and cross the conveyance direction DP of the sheet P when viewed from the front. The blowing direction of the airflow D61 is a direction indicated by an arrow D61 in FIG. 3. The blowing directions of the respective airflows D62 are directions indicated by respective arrows D62 in FIG. 3. The phrase the blowing direction of the airflow D61 and the blowing directions of the airflows D62 cross the conveyance direction DP of the sheet P refers to a route of the sheet P moving in the conveyance direction DP crossing a first imaginary line extending from the center exhaust port 61 in the blowing direction of the airflow D61 and second imaginary lines extending from the respective side exhaust ports 62 in the blowing directions of the respective airflows D62. The phrase the directions ascend downstream in the conveyance direction DP refers to the airflows D61 and D62 each flowing in a specific direction. The specific direction refers to a direction inclined upward at an acute angle relative to a downstream direction of the conveyance direction DP. Furthermore, the center exhaust port 61 and the side exhaust ports 62 are directed such that an angle G made by the conveyance direction DP of the sheet P and the blowing direction of the airflow D61 blown out from the center exhaust port 61 is larger than an angle made by the conveyance direction DP of the sheet P and the blowing directions of the airflows D62 blown out from the respective side exhaust ports 62. Specifically, the center exhaust port 61 and the side exhaust ports 62 are directed such that the angle G is greater than a first blowing angle G1 and a second blowing angle G2. The first blowing angle G1 refers to an angle made by the conveyance direction DP of the sheet P and the blowing direction of the airflow D62 blown out from one of the side exhaust ports 62. The blowing second angle G2 refers to an angle made by the conveyance direction DP of the sheet P and the blowing direction of the airflow D62 blown out from the other of the side exhaust ports 62. In other words, the blowing direction of the airflow D61 blown out from the center exhaust port 61 ascends downstream in the conveyance direction DP at an angle larger than the blowing directions of the airflows D62 blown out from the respective side exhaust ports 62. In the above configuration, the airflow D61 blown out from the center exhaust port 61 strikes on a part of the sheet surface P1 of the sheet P located closer in height to the ejection rollers 52 than a part of the sheet surface P1 thereof on which the airflows D62 blown out from the respective side exhaust ports 62 strike.

The main exit tray 54 has the inclined surface 54 a ascending downstream in the conveyance direction of the sheet P, as described above. The respective side exhaust ports 62 are directed such that the first and second blowing angles G1 and G2 between the conveyance direction DP of the sheet P and the blowing directions of the airflows D62 blown out from the respective side exhaust ports 62 are larger than a blowing angle G3 made by the inclined surface 54 a of the main exit tray 54 and the conveyance direction DP of the sheet P. Specifically, the respective side exhaust ports 62 are directed such that the first and second blowing angles G1 and G2 each are larger than the blowing angle G3. In other words, the blowing directions of the airflows D62 blown out from the respective side exhaust ports 62 ascend downstream in the conveyance direction DP at a larger angle than inclination of the inclined surface 54 a of the main exit tray 54. That is, angles of elevation of the blowing directions of the airflows D62 blown out from the respective side exhaust ports 62 are larger than that of the inclined surface 54 a of the main exit tray 54.

FIG. 7 is a perspective view of the sheet P in a situation in which the airflows DF including the airflow D61 and the airflows D62 is blown toward the sheet P. Note that the conveyance direction of the sheet P ejected by the ejection rollers 52 and the driven rollers 53 is indicated by an arrow DP in FIG. 7. FIG. 8 is an enlarged cross-sectional view of the post-printing processing unit 5 in a situation in which failure in sheet ejection occurs.

The sheet P is liable to be charged when being conveyed into the main unit 1 and the post-printing processing unit 5 of the image forming apparatus S. Accordingly, when the sheet P having the sheet surface P1 that has been electrostatically charged is ejected by the ejection rollers 52, a leading edge P2 of the sheet P is attracted to the main exit tray 54, as illustrated in FIG. 8. As a result, the sheet P cannot be stacked on the main exit tray 54 in a favorable fashion, which may cause failure such as a jam.

The duct 73 has the three exhaust ports 61 and 62 through which the airflows are blown out in the present embodiment in order to obviate failure as above. The airflows blown out from the duct 73 strike on the sheet surface P1 of the sheet P facing downward and enters a space between the sheet P and the main exit tray 54, as indicated by an arrow DF in FIG. 6.

The amount of the airflow (arrow D61 in FIG. 7) blown out from the center exhaust port 61 is larger than those of the airflows (arrows D62 in FIG. 7) blown out from the respective side exhaust ports 62. A central part of the sheet P having passed through the nip parts 56 warps upward and in the conveyance direction, as illustrated in FIG. 7. As a result, remarkable resilience can be imparted to the sheet P ejected onto the main exit tray 54 in a secure manner, thereby achieving sheet alignment in a secure manner. Furthermore, an air layer is formed between sheets P in continuous printing, with a result that adhesion (or sticking) between the sheets P can be reduced.

In particular, the airflow D61 blown out from the center exhaust port 61 ascends downstream in the conveyance direction DP at a larger angle than the airflows D62 blown out from the respective side exhaust ports 62 in the present embodiment, as illustrated in FIGS. 3 and 7. In the above configuration, the airflow D61 blown out from the center exhaust port 61 strikes on the sheet surface P1 at a steeper angle than the airflows D62. Therefore, the sheet P having passed through the nip parts 56 can readily warp. The central part of the sheet P warps upward and in the conveyance direction of the sheet P.

Furthermore, the blowing directions of the airflows D62 blown out from the respective side exhaust ports 62 ascend downstream in the conveyance direction DP at a larger angle than the inclination of the inclined surface 54 a of the main exit tray 54. In the above configuration, a situation in which the airflows D62 blown out from the respective side exhaust ports 62 strike hard on the inclined surface 54 a of the main exit tray 54 before striking on the sheet surface P1 of the sheet P can be prevented. Accordingly, airflow turbulence can hardly occur between the ejected sheet P and the main exit tray 54, thereby achieving improved sheet alignment.

Note that the first and second fans 71 and 72 each start blowing the airflows DF before the leading edge P2 of the sheet P passes through the respective nip parts 56 between the ejection rollers 52 and the driven rollers 53 in the present embodiment. Furthermore, the first and second fans 71 and 72 each stop generating the airflows DF before a trailing edge P3 of the sheet P passes through the nip parts 56. In the above configuration, the leading edge P2 of the sheet P can warp in a secure manner. The airflows strike not so hard on the trailing edge P3 of the sheet P having passed through the nip parts 56. As a result, stacking failure caused due to a lift of the trailing edge P3 of the sheet P can be avoided. The leading edge P2 of the sheet P refers to an edge of the sheet P located downstream in the conveyance direction DP. The trailing edge P3 of the sheet P refers to an edge of the sheet P located upstream in the conveyance direction DP.

As illustrated in FIG. 5, the two fans 71 and 72 are provided in the present embodiment. When the airflows of the two fans 71 and 72 are merged together in the duct 73, the airflow D61 having a large amount can be blown out from the center exhaust port 61. Even when one of the fans 71 and 72 stops driving, the airflow D61 blown out from the center exhaust port 61 can be ensured.

As illustrated in FIG. 4, the center exhaust port 61 is located below the region E between the two ejection rollers 52 a that are spaced from each other in middle in the sheet width direction W among the four ejection rollers 52. In the above configuration, the sheet P can readily warp upward by blowing the strong airflow D61 from the center exhaust port 61 between the two ejection rollers 52 a, as illustrated in FIG. 7.

The side exhaust ports 62 are located blow the respective ejection rollers 52 b disposed outside in the sheet width direction W among the four ejection rollers 52 (see FIG. 4). In the above configuration, the airflows D62 from the respective side exhaust ports 62 strike on respective specific regions of the sheet surface P1. The specific regions each refer to a region of the sheet surface P1 that is being located downstream in the conveyance direction DP of a region of the sheet surface P1 that is being nipped by a corresponding one of the ejection rollers 52 b located outside in the sheet width direction W. In other words, airflows D62 are blown from the respective side exhaust ports 62 toward regions of the sheet surface P1 that are being located downstream of the respective ejection rollers 52 in the conveyance direction DP. The regions that are being nipped by the respective ejection rollers 52 b refer in other words to regions of the sheet surface P1 that are in contact with the respective ejection rollers 52 b. In the above configuration, a situation in which the ejected sheet P is wobbled and skewed by the airflows D62 blown out from the respective side exhaust ports 62 can be prevented.

The outside edges (first and second outside edges L1 and L2) of the respective side exhaust ports 62 in the sheet width direction W are substantially aligned in the sheet width direction W with the outside edges (third and fourth outside edges Q1 and Q2) of the respective ejection rollers 52 b in the sheet width direction W that are disposed outside in the sheet width direction W. The inside edges (first and second inside edges M1 and M2) of the respective side exhaust ports 62 in the sheet width direction W are each located inside in the sheet width direction W from the inside edge (third and fourth inside edges R1 or R2) of a corresponding one of the corresponding ejection rollers 52 b in the sheet width direction W. In the above configuration, respective parts of the airflows D62 blown out from the side exhaust ports 62 flow inside in the sheet width direction W to be merged with the airflow D61 blown out from the center exhaust port 61. Accordingly, the sheet P having passed through the nip parts 56 can warp upward and in the conveyance direction in a secure manner.

One embodiment of the present disclosure has been described so far in detail. In the above configuration, remarkable resilience can be imparted to the sheet P being ejected onto the main exit tray 54, thereby achieving sheet alignment in a secure manner. Furthermore, an air layer is formed between sheets P, with a result that adhesion (or sticking) between the sheets P can be reduced. Note that the present disclosure is not limited to the above embodiment. The present disclosure can be changed for example to any of the following alterations.

(1) The above embodiment describes the post-printing processing unit 5 as an example of the sheet stacking device, which however should not be taken to limit the present disclosure. The present disclosure is adaptable to any device that can eject and stack a sheet P on which an image has been formed.

(2) The above embodiment describes the post-printing processing unit 5 having the single center exhaust port 61 and the two side exhaust ports 62, which however should not be taken to limit the present disclosure. The post-printing processing unit 5 may have a single center exhaust port 61 and four or any other plural number of side exhaust ports 62 located outside of the center exhaust port 61 in the sheet width direction W.

(3) The four ejection rollers 52 are provided in the above embodiment. However, two pairs of ejection rollers 52 and driven rollers 53 may be spaced in the front-back direction so as to nip the central part of the sheet P in the sheet width direction W. In the above configuration, it is possible that the center exhaust port 61 is located below a region between the two pairs of the rollers and the respective side exhaust ports 62 are located below the respective two pairs of the rollers.

(4) The ejection rollers 52 and the driven rollers 53 in the above embodiment correspond to the lower ejection rollers and the upper ejection rollers, respectively, in the present disclosure. However, it is possible that the ejection rollers 52 and the driven rollers 53 may be the upper ejection rollers and the lower ejection rollers, respectively, in the present disclosure. 

What is claimed is:
 1. A sheet stacking device comprising: a sheet stacking section on which a sheet is stacked; pairs of ejection rollers that eject the sheet toward the sheet stacking section; an airflow generator configured to generate airflows; and an airflow guide configured to guide the airflows toward a lower surface of the sheet ejected from the pairs of ejection rollers toward the sheet stacking section, wherein the airflow guide has: a center exhaust port that is located below the pairs of ejection rollers and from which an airflow generated by the airflow generating section is blown out toward a central part of the lower surface of the sheet in a sheet width direction perpendicular to a conveyance direction of the sheet; and paired side exhaust ports that are located on respective opposite sides of the center exhaust port in the sheet width direction and from which airflows generated by the airflow generator are blown out toward respective sides of the lower surface, and an amount of the airflow blown out from the center exhaust port is larger than those of the airflows blown out from the respective paired side exhaust ports.
 2. The sheet stacking device according to claim 1, wherein the center exhaust port and the paired side exhaust ports are directed such that a first blowing angle made by the conveyance direction and a blowing direction of the airflow blown out from the center exhaust port is larger than a second blowing angle made by the conveyance direction and a blowing direction of the airflow blown out from each of the paired side exhaust ports.
 3. The sheet stacking device according to claim 2, wherein the sheet stacking section has an inclined surface ascending downstream in the conveyance direction, and the paired side exhaust ports are directed such that the second blowing angle is larger than a third blowing angle made by the conveyance direction and the inclined surface of the sheet stacking section.
 4. The sheet stacking device according to claim 1, wherein the airflow generator starts blowing the airflows before a leading edge of the sheet passes through the pairs of ejection rollers and stops blowing the airflows before the trailing edge of the sheet passes through the pairs of ejection rollers.
 5. The sheet stacking device according to claim 1, wherein the airflow generator includes a first airflow generator and a second airflow generator, the airflow guide includes a first duct and a second duct, the first duct divides an airflow generated by the first airflow generator and guides respective divided airflows to the center exhaust port and one of the paired side exhaust ports, and the second duct divides an airflow generated by the second airflow generator and guides respective divided airflows to the center exhaust port and the other of the paired side exhaust ports.
 6. The sheet stacking device according to claim 1, wherein the pairs of ejection rollers includes: a plurality of lower ejection rollers spaced from one another in the sheet width direction; and a plurality of upper ejection rollers each disposed opposite to a corresponding one of the lower ejection rollers, the lower ejection rollers include: paired inside rollers disposed in a central part of the sheet stacking device in the sheet width direction and spaced from each other; and paired outside rollers disposed adjacently outside of the respective inside rollers in the sheet width direction, the center exhaust port is located blow a region where the paired inside rollers are located, and the paired side exhaust ports are located below the respective paired outside rollers.
 7. The sheet stacking device according to claim 6, wherein the paired side exhaust ports each have an outside edge in the sheet width direction that is substantially aligned in the sheet width direction with an outside edge of a corresponding one of the paired outside rollers in the sheet width direction, and the paired side exhaust ports each have an inside edge in the sheet width direction that is located inside in the sheet width direction from an inside edge of a corresponding one of the paired outside rollers in the sheet width direction.
 8. The sheet stacking device according to claim 6, wherein the airflows generated by the airflow generator are blown out from the respective paired side exhaust ports toward regions of the lower surface of the sheet that are being located downstream of the respective pairs of ejection rollers in the conveyance direction.
 9. An image forming apparatus comprising: the sheet stacking device according to claim 1; and an image forming section configured to form an image on the sheet. 